An important factor for any latex paint for household use is its ability to form a continuous and consistent film at ambient temperatures, the temperature at which the paint is applied. The film formation process for latex paints involves a three step process: (1) evaporation of bulk water and latex particle packing; (2) particle deformation with collapse of the interstitial voids between latex particles; and (3) the coalescence of polymer chains between the residual particles leading to a fully developed coating/film. The minimum film-forming temperature (“MFFT”) of latex is the lowest temperature at which these processes will successfully occur, i.e. result in a uniformly coalesced film. Various properties of the constituent polymers of the latex such as their glass transition temperature (Tg), particle size and particle distribution, molecular weight, and particle morphology may have a direct effect on the minimum film-formation temperature and/or film formation process of the latex. For example, soft polymers, polymers with low Tg easily deform and yield excellent film formation properties, but the resulting film is often tacky and has poor mechanical properties. Alternatively, hard polymers, polymers with high Tg or semi-crystalline structure, maintain the physical properties of a film but do not form suitable films on their own since the hard polymers do not deform easily.
Coalescing agents, also known merely as coalescent or alternatively as film-forming agent or aid, may be added to conventional water-based latex paints to reduce the paints MFFT and thereby improve the paint's film forming properties. Typical coalescents are solvents such as Texanol™ and plasticizers such as Optifilm Enhancer 400™. These agents act during the drying process to allow the latex binder particles to coalesce, join physically, and form a continuous film at desired application temperatures. The coalescents eventually either evaporate from the resin film, in the case of solvents, or incorporate into the resulting dried latex paint film, in the case of plasticizers. The concentrations of coalescents within latex paints may vary from about 0% to about 30% by weight based on the weight of the polymeric binder solids.
Unfortunately, compatibility issues may arise between the coalescent agent and the latex binders and or additives to the paint composition. These compatibility issues may impair the physical properties of the resulting film such as block, mar, water, and/or stain resistance. The coalescents may also be flammable, introducing a fire hazard to the paint system. Additionally, the evaporation of the coalescents, especially solvents, leads to unpleasant odors and growing concerns about the environmental and health hazards posed by volatile organic evaporates. Numerous environmental regulations have limited the amount of volatile organic compound (VOC) allowable in coatings. In the past few years, coating manufacturers attempting to comply with the regulations have sought to substitute low- and no-VOC coalescents in latex paint formulations without compromising physical properties and smooth film formation of the resulting paints.
Less volatile or nonvolatile coalescents are disclosed in US 2007/0167545 to Sugerman et al., which employs nonvolatile, unsaturated ethers and/or esters in combination with small proportions of low glass transition temperature (Tg) latex resins and optionally reactive amines as coalescents. US 2010/0130645 to Lynch et al., US 2009/0198002 to Zhou et al., US 2009/0151601 to Mangnus et al., US 2008/0103237 to Strepka et al., US 2005/0032954 to Brandenburger et al., and U.S. Pat. No. 4,894,406 to Smith et al. also disclose organic coalescents and coating compositions incorporating these organic coalescents that have low odor, such as fatty acid esters of ethylene glycol and/or propylene glycol, esters of adipic, glutaric, and/or succinic diacids, and a mixture of benzoates, diesters of glycols, and corresponding monesters.
Alternatively, other attempts to eliminate VOCs involve reducing or eliminating the need for coalescents altogether. Manufacturers have attempted to achieve this by incorporating more soft monomers or a blend of soft polymer (low Tg) and hard polymer (high Tg) to lower Tg and MFFT of the latex, thereby reducing the amount of coalescents needed for smooth film formation. For example, U.S. Pat. No. 5,021,469 to Langerbeins et al., U.S. Pat. No. 6,140,408 to McCarthy et al., and co-pending US 2008/0058473 to Freidzon et al. disclose multistage latex polymers with low temperature coalescence comprising a hard center and a soft outer shell.
US 2002/0132055 to Drujon et al. discloses a low temperature film-forming latex based on hydrophobic polymers having 70-90% by weight of a soft core and 10-30% by weight of a hard shell.
U.S. Pat. No. 5,344,675 to Snyder discloses a blend of at least two emulsion polymers. The first latex comprises 50-95% by weight of a soft polymer and 5-50% by weight of a hard polymer, while the second latex is not an ambient temperature film-forming polymer. U.S. Pat. No. 5,731,377 to Friel discloses a polymer blend comprising 40-80% by weight of a soft emulsion polymer and 20-60% by weight of a hard emulsion polymer.
U.S. Pat. No. 6,646,085 to Craun et al., U.S. Pat. No. 6,303,188 to Bors et al., U.S. Pat. No. 5,610,225 to Farwaha et al., and U.S. Pat. No. 6,087,437 to Farwaha et al. disclose copolymers containing soft flexible polymer units useful in paint compositions free of volatile organic coalescents.
The problem with these approaches is that latex binders with relatively large portions of soft monomers lead to poor paint properties compromising film hardness, block resistance, print resistance, dirt pick-up resistance, and/or mar resistance. Thus, there still remains a need for a latex paint composition eliminating external coalescents for adequate film formation, while maintaining desirable paint properties.